Method and system for storing memory compressed data onto memory compressed disks

ABSTRACT

A method (and system) of storing information, includes storing main memory compressed information onto a memory compressed disk, where pages are stored and retrieved individually, without decompressing the main memory compressed information.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a divisional application of U.S. patentapplication Ser. No. 10/214,171, which was filed on Aug. 8, 2002.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to compressed memory andcompressed disk systems, and, more specifically, to a method fortransferring memory compressed data to and from the disk adapter, andfor storing memory compressed data on disk, thereby obviating the needto decompress and recompress data when transferring between memory anddisk, and thus saving time and bandwidth in the system.

2. Description of the Related Art

Main Memory Compression

An emerging development in computer organization is the use of datacompression in a computer system's main memory, where the data in themain memory is stored in a compressed format. For example, FIG. 1 showsthe overall structure of an exemplary computer system 100 implementing acompressed main memory 110.

In FIG. 1, a central processing unit (CPU) 102 reads data to and writesdata from a cache 104. Cache “misses” and “stores” result in reads andwrites to a compressed main memory 110 by a compression controller 106.

That is, typically the CPU will obtain the data from the cache 104(e.g., assuming a cache “hit”), whereas on a cache “miss” the CPU willobtain the data from the main memory 110 through the compressioncontroller 106. Such data would be retrieved to the CPU and then thedata would be stored to the cache 104 (e.g., typically a so-called“store-through” cache, which is one of the most common cachingtechniques).

The compressed main memory 108 is typically divided into a number oflogically fixed-size segments (e.g., segments spanning a range ofcontiguous memory addresses). These logical segments are the units ofcompression and are referred to as “memory lines”. Memory lines arephysically stored in a compressed format, and therefore occupy avariable number of physical memory locations. There are several proposedcomputer system designs where the contents of main memory is compressed,including U.S. Pat. No. 5,761,536, entitled “System and Method forReducing Memory Fragmentation by Assigning Remainders to Share MemoryBlocks on a Best Fit Basis” and Franaszek et al., “Design and Analysisof Internal

Organizations for Compressed Random Access Memories”, IBM ResearchReport RC 21146, IBM Watson Research Center, Mar. 30, 1998 (revised Oct.9, 1998).

In a paged memory system, the unit of memory management is a “Page”,usually consisting of 4096 bytes having consecutive real addresses,aligned on a 4096 byte boundary. The unit of compression can be any sizeand in some systems the unit of compression is a page.

To simplify management of the compressed memory system, it is better toselect the size of the unit of compression (called a “line”) so that theuncompressed size of a page is a multiple of the uncompressed size of aline. A page is then divided into a fixed number of lines, and the bytesof a line are compressed and decompressed together. Hereinbelow, it willbe assumed that an uncompressed line is 1024 bytes long.

Compressing a line produces a variable-size compressed line (e.g.,different lines will compress to different numbers of bytes). Thecompressed size of a line is an important parameter, which is needed forphysical memory management. If a line that does not compress to lessthan its original size is left uncompressed, one needs (1+log(n)) bitsto describe the size of the compressed line, where n is the originalsize in number of bytes, and the logarithm is base 2.

To simplify management, the size of a compressed line can be constrainedto be a multiple of a fixed number of bytes, for instance by “padding”the compressed line with zeros. The term “granule” is used to denote theatomic size of compressed data. The granule size should be a divider ofthe line size.

FIG. 2 shows, in more detail, an architecture 200 of a compressed mainmemory system 210. The architecture 200 includes a cache 240, containingcache lines 241 and a cache directory 242, the compression controller260, containing a compressor 261 and a decompressor 262, and compressedmain memory 210. The compressed main memory 210 includes a directory 220and a number of fixed sized blocks 230, also called “memory sectors”.The above exemplary format and organization is what is referred to as“main memory compressed data.”

As shown in FIG. 2, for a read from the compressed memory to the cache,the decompressor 262 is employed, whereas on a write from the cache tothe compressed main memory, the line must be compressed via thecompressor 261. As known, lines in the cache are similarly held in themain memory. Along these lines, if a line must be evicted from the cache(e.g., the cache capacity is exhausted), then the cache line beingevicted is written back to the compressed main memory 210 via thecompressor 261 if modified.

If a cache line associated with address A is not found in the cache 240,then this address A 270 is used to index into the directory D 220. Thisis different from a conventional computer system where the address isused to directly reference a location in the memory M 210.

Each directory entry contains information which allows the associatedcache line to be retrieved. The units of compressed data referred to bydirectory entries in D 220 may correspond to cache lines 241, or theunit of compression may be larger than a cache line in which casemultiple cache lines would be compressed together into one unit ofcompression.

The directory entry 221 for line 1 associated with address A1 271 is fora line which has been compressed to a degree in which the compressedline can be stored entirely within the directory entry. Lines A2 272, A3273, and A4 274 in FIG. 2 do not compress sufficiently to be stored in adirectory entry, and therefore they are stored in an integer number ofmemory sectors.

The directory entry 222 for line 2 associated with address A2 272 is fora line which is stored in compressed format using a first full memorysector 231 and second partially filled memory sector 232.

Finally, the directory entries 223 and 224 for line 3 and line 4associated with addresses A3 273 and A4 274 are for lines stored incompressed formats using a number of full memory sectors (2 sectors 233and 234 for line 3, and no sector for line 4) and in which theremainders of the two compressed lines have been combined in block 235.Memory-sector sharing by different lines within a page (or across pages)is done to reduce fragmentation, and in this situation, lines sharing amemory sector are called roommates.

FIG. 3 shows some examples of directory (220) entry formats, which canbe used in the directory 220 of the main memory system 210 of FIG. 2.For this example, it is assumed that the memory sectors 230 of FIG. 2are of size 256 bytes, and that the cache lines 241 of FIG. 2 are ofsize 1024 bytes. Hence, lines can be stored in an uncompressed formatusing four memory sectors.

For this example, directory entries of size 16 bytes are used, in whichthe first byte includes a number of flags. The contents of the firstbyte 301 determines the format of the remainder of the directory entry.

A flag bit 302 specifies whether the line is stored in compressed oruncompressed format. If stored in uncompressed format, then theremainder of the directory entry is interpreted as for line 1 306, inwhich four 30-bit addresses give the addresses in memory of the fourmemory sectors containing the line.

If stored in a compressed format, then a flag bit 303 indicates whetherthe compressed line is stored entirely within the directory entry. Ifso, then the format of the directory entry is as for line 3 308, inwhich up to 120 bits of compressed data are stored. Otherwise, forcompressed lines longer than 120 bits, the formats shown for line 1 306or line 2 307 may be used.

In the case of the line 1 306 format, additional flag bits 304 specifythe number of blocks used to store the compressed line, from one to four30-bit addresses specify the locations of the sectors, and finally thesize of the remainder, or fragment, of the compressed line stored in thelast memory sector (in units of 32 bytes), together with a bitindicating whether the fragment is stored at the beginning or at the endof the memory sector, is given by four fragment information bits 305.

Directory entry format 307 illustrates an alternative format in whichpart of the compressed line is stored in the directory entry (to reducedecompression latency). In this case, addresses to only the first andlast sectors used to store the remaining part of the compressed line arestored in the directory entry, with intervening sectors (if any) foundusing a linked list technique. That is, each sector used to store thecompressed line has, if required, a pointer field containing the addressof the next memory sector used to store the given compressed line.

Thus, many different formats may be used in the directory 220 for thecompressed main memory according to the decompression/compression of thedata.

Disk Compression

Another problem arises in disk systems in going from the disk to themain memory and trying to individually access compressed pages on thedisk as opposed to accessing compressed data in the main memory. Indeed,typically the disk and main memory employ two different compressionsystems having different formats. As a result, compressed data from themain memory must be decompressed and then sent to the disk system whereit is recompressed again, and vice versa. Thus, there has been nofacility to take compressed data from the disk and place it directlyonto the main memory, where the compressed data can then be referencedas described above as compressed main memory data.

In some disk systems, such as the IBM AS/400 DASD® adapter, pages arestored in compressed form to improve the price/capacity ratio.Typically, individual pages (or groups of pages) are compressed into avariable number of fixed-length blocks, called “disk sectors”.

FIG. 4 shows the structure of the UO processor (IOP) 400 and how it isattached to the rest of the system, and more specifically is ahigh-level representation of an Input/output adaptor (IOA) 405 and theinput/output processor 400.

Generally, it is shown that one side is connected back to the system andthe other side will be connected to the disk system, and the IOP 400generally has its own compressor/decompressor 403. That is, data that isbeing stored on the disks will be compressed in the IOP since theinformation from the main memory does not come already compressed fromthe main memory.

The operation of the IOP 400 is controlled by the IOP processor complex402, which communicates with the main system over a system bus 406through a system bus to IOA bus adapter 401. The IOP 400 has a localmemory 404. If the IOP supports compression in hardware, it contains acompressor/decompressor subsystem 403. The IOP controls one or more IOAadapters 405, which provides an interface with the direct access storagedisk (DASD) over a DASD bus 407.

In a compressed disk, the unit of compression is typically a page (e.g.4096 bytes), which is also the atomic unit of UO. Physical space ondisks is managed in atomic units called sectors. A disk sector typicallycontains 512 bytes of data and can contain additional information suchas cyclic redundancy code (CRC) bytes and system headers.

When a page is written to a compressed disk, it is first compressed, andthe resulting compressed page is stored in an integer number of disksectors.

Several approaches to managing and organizing compressed disks arepossible, which rely on different directory data structures.

From the description of the conventional techniques, it is clear thatthe organization of a compressed disk and of a compressed memory areusually very different. This is the case, for instance, when the disksubsystem is separate from the computing units (as in Enterprise Storageservers such as SHARK® or EMC® disk servers), or when the twotechnologies are developed separately (as would be the case for AS/400®machines supporting memory compression and using the current diskadapter.)

Since the structures that have been created to support memorycompression and disk compression are quite different, someinefficiencies arise when a system supporting memory compressionoperates with a compressed disk.

For example, when data is transferred from memory to disk and from diskto memory, the data must first be decompressed, then transferred, andthen recompressed, as mentioned above.

In principle, data could be transmitted in compressed form to and fromthe disk. This would not only obviate the need to perform additionalcompressions and decompressions, but it would also effectively increasethe bandwidth of the links.

Additionally, if the data is transmitted from memory to disk adapter incompressed format, then the disk adapter would know in advance the sizeof the data, and would be able to make better storage managementdecisions. In contrast, in current compressed disk systems, the diskadapter allocates storage based on how it expects the data to compress.

However, transmitting such compressed data directly to disk withoutadditional and costly steps of decompression and recompression requireschanges to the UO operations and the system structure. These changes arethe subject of the present invention.

SUMMARY OF THE INVENTION

In view of the foregoing and other problems, drawbacks, anddisadvantages of the conventional methods and structures, an object ofthe present invention is to provide a method and structure in whichreduced latency in UO operations is achieved.

Another object is to provide a method and system in which data can betransferred to and from disk without having to be buffered while waitingfor compression/decompression.

Another object is to provide a method and system in which increasedbandwidth is provided between main memory and disk, by a factor equal tothe compression ratio in main memory.

Yet another object is to provide a method and system in which acompressed disk can be used as a paging system for a compressed memorysystem.

Yet another object is to provide a method and system which do not relyon specific features of the compressed disk organization, and can beused with any existing compressed disk scheme.

In a first aspect of the present invention, a method (and system) ofstoring information, includes storing main memory compressed informationonto a memory compressed disk, where pages are stored and retrievedindividually, without decompressing the main memory compressedinformation.

In a second aspect of the present invention, a method (and system) foruse in a computer system supporting main memory compression, connectedto a storage system supporting compression, includes storing memorycompressed data to compressed disk, and retrieving memory compresseddata from disk. It is noted that “data” refers to the generic content ofmemory, and includes program code.

With the unique and unobvious aspects of the present invention, a method(and system) are provided for storing memory-compressed data directly todisk.

Further, the invention results in reduced latency in UO operations.Indeed, the data can be transferred to and from disk without having tobe buffered while waiting for compression/decompression.

Further, reduced resource consumption occurs at the disk adapter. Thatis, the data does not need to be buffered while waiting forcompression/decompression, and there is no need to manage the queue ofpages waiting to be compressed/decompressed, etc.

Additionally, increased bandwidth is possible between main memory anddisk, by a factor equal to the compression ratio in main memory.

Furthermore, better algorithms are provided for data placement on disk.

That is, since the size of the compressed data is known, better storagedecisions can be made about how to place the compressed data on disk

Additionally, with the invention, a compressed disk can be used as apaging system for a compressed memory system. That is, in a compressedmemory system without compressed secondary store, pages swapped to diskby the operating system are decompressed while the UO operation isexecuted.

Further, the present invention does not rely on specific features of thecompressed disk organization, and can be used with any existingcompressed disk scheme. Indeed, the invention can be used by existinghardware systems (e.g., the exemplary hardware systems of FIGS. 1 and 4)and does not require any substantial modification to the existinghardware.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other purposes, aspects and advantages will be betterunderstood from the following detailed description of a preferredembodiment of the invention with reference to the drawings, in which:

FIG. 1 illustrates a structure of a computer system 100 using acompressed main memory 110;

FIG. 2 illustrates a compressed main memory system organization 200;

FIG. 3 illustrates a directory entry format of the compressed mainmemory 220;

FIG. 4 illustrates an YO Processor (IOP) 400 and UO Adapter (IOA) 405;

FIG. 5 illustrates a flowchart of a method 500 for a communicationprotocol between an IOP (e.g., 400) and an operating system (OS) duringa disk write, for a system supporting direct memory access (DMA)transfers from and to compressed memory, according to the presentinvention;

FIG. 6 illustrates an exemplary storage format 600 to store data from aninteger number memory sectors onto a disk into each disk sector,according to the present invention; and

FIG. 7 illustrates a signal bearing medium 700 (e.g., storage medium)for storing steps of a program of a method according to the presentinvention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

Referring now to the drawings, and more particularly to FIGS. 1-7, thereare shown preferred embodiments of the method and structures accordingto the present invention.

Preferred Embodiment

In a preferred embodiment, it is assumed that cache lines and memorylines have the same size (e.g., 1024 bytes), that memory granules are 64bytes long, and that memory sectors are 256 bytes long and are alignedon 256-byte boundaries.

Obviously, as would be evident to one of ordinary skill in the arttaking the present application as a whole, the invention is not limitedto this particular choice of methods.

The invention has several main components, including how to transmitmemory-compressed data from the processor to the IOP, and between theIOP and the disk, and how to store memory-compressed data on disk. Thesecomponents will be discussed in turn below.

Transmitting Memory-Compressed Data to and from the I/O Processor/I/OAdapter (IOP/IOA)

The transfer of data between the processor and the IOA/IOP and betweenthe IOA/IOP and the disk depends on the architecture of the computingsystem.

In a preferred embodiment, the architecture of the computer supports aninterprocessor communication protocol that allows data to be transferredbetween different nodes in compressed format.

In one embodiment, the memory controller provides support for NUMA (nonuniform memory access).

In another embodiment, the memory controller supports message passingbetween processors or processor clusters. In both embodiments, data istransferred in a compressed format. The atomic unit of transfer is theatomic unit of compression or a multiple thereof, and the informationrequired to decompress the data is transmitted before the compresseddata. This information, for example, includes the Flags 301 of FIG. 3.

In these preferred embodiments, the operating system driver in thecomputer and the code in the UO Processor, or IOP, would communicate thesame information (e.g., directory information and compressed memorysectors) that is transmitted when processors transfer data betweenthemselves, as described above.

In another preferred embodiment, the architecture allows direct memoryaccess (DMA) to compressed memory.

FIG. 5 depicts a flowchart of a method 500 illustrating a disk writeoperation (e.g., storage operation) utilizing the present invention.Again, it is noted that the invention can be applied to the existinghardware, to make the hardware more efficient.

In step 501, the operating system sends to the I/O Processor the entiredirectory contents (all lines) associated with the page to be written.The IOP iterates over each line contained in the page (step 502), untilthere are no more lines (step 510) at which time the process ends. It isnoted again that “line” refers to however the line is compressed (e.g.,whatever unit is employed for compression).

For each line, the IOP parses the directory flags (e.g., flags 301 shownin FIG. 3 including “compressed/uncompressed”, “stored in directory”,etc.) in step 503 and tries to understand (determine) how the data hasbeen stored (e.g., what the format is), and checks whether thecompressed line is contained in the directory in step 504.

In the inventive method, it is generally assumed that the line size islarger than the unit of compression, and the disk is looking on a pagebasis. Thus, it will be determined which lines are in the page and thenobtain those directories. Then, the system must determine if the line isstored in the directory, and then either the system will find the datain the directory or retrieve the data out of the memory sectors. In anyevent, upon completion, the system will have the data for each line.

Thus, returning to the flowchart of FIG. 5, if the compressed line isstored entirely in (compresses to) the directory, in step 505 the IOPextracts it from the directory entry, and the next line is accessed instep 502.

Otherwise (e.g., if the line does not compress to the directory in step504; a “NO” in step 504), in step 506 the IOP iterates over the memorysectors containing the compressed line. The IOP extracts from thedirectory entry the address of each sector in step 507, and reads thesector from memory through a DMA operation in step 508.

Then, the process loops back to step 506 if there are more memorysector(s). Such a process iterates until all of the data has beenretrieved for a particular line, from the memory sector(s).

If the last sector is shared between different memory lines (e.g., thecase of “roommating” in which a last portion of information is in a lastsector being shared), then in step 509 the IOP extracts data from thelast sector. It is noted that, in such a case, it would be advantageousto determine where the data is located in the last sector (e.g., thebeginning of the sector or the end of the sector). Such a location wouldbe indicated preferably by a directory flag 305 as shown in FIG. 3.

In a different embodiment, step 509 is not performed, and the sharedsector is not disassembled. In both embodiments, a shared sector is readonly once from memory. When the last line has been read from memory, thetransfer protocol terminates in step 510.

Thus, the invention provides an efficient way of storing data that hasalready been compressed in the memory, onto the disk.

Regarding a read operation from disk, the above operation may be similarif the data compression/storage format used was the same as in thememory (e.g., the format of FIG. 6 described below). By the same token,if the data was compressed differently, then the above operation wouldbe somewhat different since the data would be compressed differently.

That is, during a read, the operating system driver communicates to theIOP n addresses of unused segments, where n is the maximum number ofsegments required to store the data (e.g., in this example, 16 perpage). These segments are also removed from the list of free segments.

The IOA reads the data from the disk and writes the segments in theindicated locations, and at the same time re-assembles the directoryinformation from the contents of the disk and the physical addresses ofthe segments.

When the transfer is completed, the directory information iscommunicated to the operating system driver, which updates thecompressed may be smaller than n, any unused segments are returned tothe free list.

Storing Memory-Compressed Data onto a Compressed Disk

FIG. 6 illustrates a method of storing the data onto a disk in the disksectors 601, 602, 603.

The unit of UO in typical computer systems is either a disk sector or apage. In the AS/400® compressed DASD, the unit of UO is a page. WindowsNT® and Windows 2000® also read and write data in pages.

The IOP of a compressed DASD compresses 4K of data into a variablenumber of bytes, and stores them into a certain number of sectors. Inthe preferred embodiment, which for example uses the AS/400® compressedDASD, a page is stored in a variable number of disk sectors, and twodifferent pages cannot share a disk sector. Compressed pages are alsostored in contiguous disk sectors. Pages that do not compress to apredetermined amount (e.g., ⅞ or less) of their original size, includingsome header information which is stored with the data, are storeduncompressed in 9 disk sectors: 8 for the data and 1 for the systemheader.

Generally, the units of compression may not be the same on the disk andon the memory. Hence, the exemplary scheme of FIG. 6 may be used. In theexemplary scheme, each disk sector can hold 512 bytes, and thus canstore two memory sectors having 256 bytes. In this invention, twomethods are proposed for storing memory-compressed data in a variablenumber of disk sectors.

First Method

The first method includes storing the Compressed System Header,Compressed Data Control and Pads (if needed, as it is the case in theAS/400® DASD) together with the directory information in a 256B memorysector, and storing this memory sector together with the memory sectorsof the compressed lines into the data portion of disk sectors.

Since, in the preferred embodiment, a disk sector contains 512 bytes,two (2) memory sectors can be stored in each disk sector. A conventionis followed for the order in which memory segments are stored in disksegments, so that the IOA knows where shared memory sectors are storedon disk.

In the example illustrated in FIG. 6, disk sectors are depicted asthick-lined rectangles, and memory sectors are depicted as thin-linedrectangles.

Disk sector 601 stores a memory sector 610 containing a compressedsystem header, compressed data control and pads, and directoryinformation for the lines in the page, and the first memory sector 611containing compressed data belonging to line 1 of the stored page. Inthis example, memory line 1 compresses to two memory sectors, the secondof which (612) is stored at the beginning of disk sector 602. The secondpart of disk sector 602 contains the first memory sector used by line 2(613). The last disk sector 603 used to store the compressed pagecontains the last memory sector (614 in FIG. 6).

The advantage of the above-described solution is its simplicity. Thatis, memory segments are left unmodified, and are read and written inexactly the same format as used in main memory.

The disadvantage is increased fragmentation due to three factors. Forexample, if lines are not roommates, their last memory segment ispartially filled. Additionally, the additional memory segment containingthe directory information and the system header is mostly empty.

Finally, if the number of memory sectors used by a page and the header,control, and directory information is odd, one of the disk sectors isonly half utilized, as depicted in FIG. 6 (sector 603).

Second Method

The second method includes organizing the data as follows:

Compressed System Header, Compressed Data Control, Pad,(page_size/line_size)*log 2(line_size/granule_size) bits to describe thenumber of granules used by all the compressed lines, and a counter arestored together in a variable number of granules. These granules aretermed “header granules”. The counter contains the number of headergranules. The format is such that the IOA can easily recover the data.The granules that make up each compressed line are extracted from thememory segments by the IOA, sequentially concatenated to the headergranules, and stored in an integer number of disk sectors.

Upon reading from disk, the IOA reassembles granules into memorysegments according to the memory compression format.

The advantage of this solution is improved fragmentation, which is nowon average equal to 2.5 times the size of a granule (before organizingthe data into a fixed number of segments). A second potential advantageresults from the possibility of better reorganizing the roommates whenthe data is read from disk.

The disadvantage is a slightly increased complexity of the code at theIOA.

In addition to the environment described above, a different aspect ofthe invention includes a computer-implemented method for performing theabove method. As an example, this method may be implemented in theparticular environment discussed above. Such a method may beimplemented, for example, by operating a computer, as embodied by adigital data processing apparatus, to execute a sequence ofmachine-readable instructions. These instructions may reside in varioustypes of signal-bearing media.

Thus, this aspect of the present invention is directed to a programmedproduct, comprising signal-bearing media tangibly embodying a program ofmachine-readable instructions executable by a digital data processorincorporating a CPU and hardware above, to perform the method of theinvention.

This signal-bearing media may include, for example, a RAM containedwithin the CPU, as represented by the fast-access storage for example.Alternatively, the instructions may be contained in anothersignal-bearing media, such as a magnetic data storage diskette 700 (FIG.7), directly or indirectly accessible by the CPU.

Whether contained in the diskette 700, the computer/CPU, or elsewhere,the instructions may be stored on a variety of machine-readable datastorage media, such as DASD storage (e.g., a conventional “hard drive”or a RAID array), magnetic tape, electronic read-only memory (e.g., ROM,EPROM, or EEPROM), an optical storage device (e.g. CD-ROM, WORM, DVD,digital optical tape, etc.), paper “punch” cards, or other suitablesignal-bearing media including transmission media such as digital andanalog and communication links and wireless. In an illustrativeembodiment of the invention, the machine-readable instructions maycomprise software object code, compiled from a language such as “C”,etc.

Thus, with the invention, numerous advantages are provided includinglower required bandwidth, less disk space, less space required in thedisk cache, and better utilization of the memory compressor/decompressor(not required to store/retrieve pages from disk).

While the invention has been described in terms of several preferredembodiments, those skilled in the art will recognize that the inventioncan be practiced with modification within the spirit and scope of theappended claims.

Further, it is noted that, Applicant's intent is to encompassequivalents of all claim elements, even if amended later duringprosecution.

1. A method of transmitting compressed data from a main memory to aninput/output adaptor (IOA)/input/output processor (IOP), said methodcomprising: sending compressed memory directory information to theIOA/IOP; interpreting the compressed memory directory information at theIOA/IOP to obtain a physical memory locations of memory sectors in saidmain memory used by the desired data; and copying a content of saidmemory sectors to the IOA/IOP using a direct memory access (DMA)operation, without decompressing the data.
 2. The method of claim 1,wherein said main memory and said disk are incorporated into a computersystem supporting memory compression and wherein data is stored on saiddisk in a different compressed format from that of data stored in saidmain memory, and wherein the IOA (input/output adaptor)/IOP(input/output processor) selectively reads from and writes to mainmemory through said DMA operation.
 3. The method of claim 1, furthercomprising: locking directory entries of the desired data before sendingthem to the IOA/IOP; and unlocking the directory entries of the desireddata after all corresponding ones of the memory sectors have been copiedto the IOA/IOP.
 4. The method of claim 1, wherein the data is stored inthe main memory in an uncompressed format.
 5. The method of claim 1,wherein at least part of the memory compressed data is stored in amemory compression directory.
 6. A system for transmitting compresseddata from a main memory to an input/output adaptor (IOA)/input/outputprocessor (IOP), said system comprising: means for sending compressedmemory directory information to the IOA/IOP; means for interpreting thecompressed memory directory information at the IOA/IOP to obtainphysical memory locations of memory sectors used by the desired data;and means for copying a content of said memory sectors to the IOA/IOPusing a direct memory access (DMA) operation, without decompressing thedata.
 7. The system of claim 6, wherein said main memory and said diskare incorporated into a computer system supporting memory compression,wherein data is stored on said disk in a different compressed formatfrom that of data stored in said main memory, and wherein the IOA/IOPselectively reads from and writes to main memory through said DMAoperation.
 8. A signal-bearing medium tangibly embodying a program ofmachine-readable instructions executable by a digital processingapparatus to perform a method of transmitting compressed data from amain memory to an input/output adaptor (IOA)/input/output processor(IOP), said method comprising: sending compressed memory directoryinformation to the IOA/IOP; interpreting the compressed memory directoryinformation at the IOA/IOP to obtain a physical memory locations ofmemory sectors in said main memory used by the desired data; and copyinga content of said memory sectors to the IOA/IOP using a direct memoryaccess (DMA) operation, without decompressing the data.